Caption: Caesium clock vacuum chamber. This atomic clock, also known as a frequency standard, uses the oscillation of caesium atoms to measure the exact length of one second. The current basis for the international definition of time is the caesium atomic clock, where one second is about 9193 million oscillations of caesium-133 atoms. Photographed at the All-Russian Scientific Research Institute of Physical-Technical and Radiotechnical Measurements, Mendeleevo, Moskovskaya oblast, Russia.